1. Field of the Invention
The present invention relates to a large aperture lens optical system.
2. Description of the Related Art
Recently, the demand for single lens reflex cameras has significantly increased. Much attention has been paid to digital single lens reflex cameras employing an image sensor, such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), instead of film cameras. Digital single lens reflex cameras are highly pixelized according to the demand of users and the development of image sensor technology. Thus, an interchangeable lens used in digital single lens reflex cameras needs to have excellent optical performance.
A single lens reflex camera includes a reflective mirror which bends the light path by 90 degrees before photographing so that a user can recognize a viewing angle through a view finder and moves upwards by 45 degrees during photographing so that light directly reaches an imaging device without contacting the reflective mirror. Due to these characteristics of the single lens reflex camera, an interchangeable lens needs to have a long back focal distance in order not to intersect the path of the reflective mirror. A double Gauss type optical system is widely used in order to manufacture a standard short focus lens having a long back focal distance and a viewing angle of about 40 to 50 degrees.
By using the double Gauss type optical system, longitudinal spherical aberration, which is the most serious problem in a large aperture lens system, can be efficiently reduced, and a long back focal distance that is essential for the single lens reflex camera can be secured. In addition, a large sagittal coma aberration, which is the most serious problem of the double Gauss type optical system, can be corrected by using a material having a high refractive index and employing an aspherical lens. However, the double Gauss type optical system does not provide sufficiently high optical performance required for the highly pixelized digital single lens reflex camera.
In addition, according to the double Gauss type optical system, the entire lens group moves in order to correct the image plane movement in accordance with a change in the distance from the object. However, when the method of correcting image plane movement is used in a large aperture lens having an aperture ratio of 1:1.2-1.5, longitudinal spherical aberration is significantly changed as the distance from the object is changed from infinity to a short distance, and thus it is difficult to secure an excellent optical performance in the entire range of distance from the object.